1Department of Mechanical Engineering, School of Science and ... thermal expansion (CTE) is high. .... fibers with a high aspect ratio increased in the parallel.
Materials Transactions, Vol. 49, No. 11 (2008) pp. 2664 to 2669 #2008 Japan Foundary Engineering Society
Microstructure and Thermal Properties of Squeeze Cast Aluminum Alloy Composite Reinforced with Short Potassium Titanate Fiber* Kazunori Asano1 , Hiroyuki Yoneda1 and Yasuyuki Agari2 1 2
Department of Mechanical Engineering, School of Science and Engineering, Kinki University, Higashiosaka 577-8502, Japan Osaka Municipal Technical Research Institute, Osaka 536-8553, Japan
Aluminum alloy composites reinforced with the short potassium titanate fibers were fabricated to obtain a material having a low thermal expansion rate and good machinability. The composites were fabricated by the squeeze casting. The microstructure, thermal conductivity and thermal expansion behavior of the composites were investigated. Optical microscopy revealed that the fibers were homogeneously distributed in the alloy. However, the fibers were somewhat in a random planar arrangement parallel to the pressed plane when the fiber volume fraction was high. This is due to the forming of the preform by pressing the top and bottom of it. The composites were easily machined using both super alloy and diamond cutting tools. The thermal conductivity of the composite decreased as the fiber volume fraction increased. At the higher volume fraction, the thermal conductivity of the composite in the direction parallel to the pressed plane was higher than that in the transverse direction due to the random planar arrangement of the fibers. The thermal conductivity can be roughly estimated by Landauer’s model. The average thermal expansion coefficient of the composite decreased as the fiber volume fraction increased. The difference in the thermal expansion coefficient between the parallel and transverse directions to the pressed plane was slight, and the experimental values were in good agreement with the theoretical values calculated using the rule of mixture. [doi:10.2320/matertrans.F-MRA2008833] (Received April 4, 2008; Accepted July 31, 2008; Published October 25, 2008) Keywords: aluminum, potassium titanate fiber, composite, squeeze casting, microstructure, hardness, thermal conductivity, thermal expansion
1.
Introduction
From the viewpoint of energy savings or protection of the environment, the reduction in the weight and size of the industrial products, such as automobile parts, have recently been promoted, and incidentally, the use of light metals, such as the aluminum alloys, has increased instead of the ferrous alloys, such as the steel and cast iron. However, the application of the aluminum alloy for precision instruments which require a small change in dimensions versus a temperature change is difficult because its coefficient of the thermal expansion (CTE) is high. The reinforcement with ceramic having a small CTE is a possible technique to reduce the thermal expansion of the aluminum alloy. Therefore, ceramics such as alumina fiber,1) SiC whisker,2) and aluminum borate whisker3) have been used as the reinforcement of the aluminum alloy. Potassium titanate, one such ceramic, has a low thermal expansion. In addition, the hardness of the potassium titanate is lower than that of the other ceramics. Therefore, the reinforcement with the potassium titanate would not inhibit the machinability of the aluminum alloy. Although several investigations have been conducted on the potassium titanate whisker-reinforced aluminum alloy composites,4–8) the whiskers are considered harmful to the respiratory organs.9) The short potassium titanate fiber, whose diameter and length are greater than the whisker, was developed to reduce this concern.10) Based on these findings, a new aluminum alloy matrix composite, which has a low thermal expansion rate, good machinability and harmless to the human body, would be obtained by the reinforcement with the short potassium titanate fiber. However, there have been no reports on the microstructure and thermal properties of the short potassium titanate fiberreinforced aluminum alloy composites. In the present study, *This Paper was Originally Published in Japanese in J. JFS 80 (2008) 8–14.
the short potassium titanate fiber-reinforced aluminum alloy composites having various fiber volume fractions were fabricated by the squeeze casting. The microstructure, thermal conductivity and thermal expansion behavior of the composites were investigated. 2.
Experimental Procedure
The AC4A aluminum alloy with the chemical composition shown in Table 1 was used as the matrix metal. The short potassium titanate fibers (TXAX-A, Kubota Co.) were used as the reinforcement. Figure 1 is a SEM micrograph of the fibers, showing that their average diameter is 30 mm and average length is 150 mm. The chemical composition and properties of the potassium titanate fiber10) along with the properties of the AC4A alloy11) are shown in Table 2. The Table 1
Chemical composition of AC4A aluminum alloy (mass%).
Si
Mg
Mn
Fe
Cu
Ca
Cr
Pb
Ti
Al
9.00
0.51
0.38
0.17
0.02
0.02
0.01
0.01
0.01
Bal.
Fig. 1
SEM micrograph of potassium titanate fibers.
Microstructure and Thermal Properties of Squeeze Cast Aluminum Alloy Composite Reinforced with Short Potassium Titanate Fiber
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Table 2 Properties of potassium titanate fiber10Þ and AC4A aluminum alloy.11Þ Potassium titanate fiber
AC4A aluminum alloy
Composition
K2 O6TiO2
Average diameter (mm)
30
Average length (mm)
150
Density (Mg/m3 )
3.53
2.68
Melting point (K)
1583–1623
868
Specific heat (J/(kgK))
920 (at 293 K)
963 (at 293 K)
Thermal conductivity (W/(mK))
1.7 (at 293 K)
138 (at 298 K)
Thermal expansion coefficient (106 /K) (293 K–373 K)
6.8
21
Vickers hardness
160–200
65
10 µ m Fig. 2
Vickers hardness of the fiber is 160–200 HV, which is considerably lower than that of alumina (1500–2000 HV12)). The preforms were fabricated as follows. The fibers were dispersed using careful agitation in an aqueous medium containing polyvinyl alcohol (PVA) as an organic binder and Al2 O3 sol as an inorganic binder. Dewatering was conducted by press forming, followed by drying at 373 K for 3 h to drive off any residual free water and to obtain the bond strength among fibers due to the PVA. After drying, the preform was sintered at 1173 K for 1 h to burn off the PVA and generate strength in the Al2 O3 binder. The preform had a 50 mm diameter and was 20 mm thick. The fiber volume fraction in the composite (hereinafter Vf ) was set to 25, 35 and 45 vol%. Figure 2 shows the appearance and SEM micrograph of a preform (Vf ¼ 35 vol%). In the preform, the fibers were oriented in a random configuration. The composite was fabricated by a squeeze casting process. Figure 3(a) is a schematic illustration of the squeeze casting. The preform was horizontally placed in the mold, and the AC4A alloy melt (1073 K) was poured into the mold (673 K). Pressure (40 MPa) was quickly applied and maintained until the solidification was complete. The microstructure and hardness of the composites were examined. The hardness was measured using a Vickers
hardness tester under a load of 98 N for 15 s. The thermal conductivity and thermal expansion rate of the composites were measured in two directions: direction parallel to the pressed plane (hereinafter P direction) and perpendicular to the pressed plane (hereinafter V direction), as illustrated in Fig. 3(b). The disc test pieces of 10 mm diameter and 1.5 mm thickness for the thermal conductivity measurements were machined from the composite as illustrated in Fig. 3(b). After measuring the thermal diffusivity using the laser flash technique, the thermal diffusivity was substituted into eq. (1) to calculate the thermal conductivity: ¼ a Cp
AC4A alloy melt
Cp ¼ Cf Vf þ Cm ð1 Vf Þ
(a) Squeeze casting
ð2Þ
where Cp , Cf , and Cm is the specific heat of the composite, fiber, and matrix, respectively. In the present study, the specific heat of potassium titanate (920 J/(kgK)10)) and the AC4A alloy (963 J/(kgK)11)) were respectively used for the Cf and Cm values. was measured using the Archimedian principle. The laser flash was performed three times per specimen at 300 K. The bar test pieces of 5 mm diameter and 17 mm length for the thermal expansion test were machined from the composite as illustrated in Fig. 3(b). The thermal expansion rate from 310 to 780 K was measured using the dilatometer at atmospheric pressure. The heating and cooling
Thermal expansion test pieces
Alloy part (unreinforced part)
P direction V direction
Thermal conductivity test pieces P direction V direction
Mold Preform
ð1Þ
where is the thermal conductivity (W/(mK)), a is the thermal diffusivity, Cp is the specific heat (J/(kgK)), and is the density (Mg/m3 ). The specific heat of the composite was calculated using eq. (2):
Pressure Plunger
Appearance and SEM micrograph of a preform (Vf ¼ 35 vol%).
Composite part
(b) Test pieces
Fig. 3 Schematic illustrations of squeeze casting and test pieces in a composite.
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K. Asano, H. Yoneda and Y. Agari
Alloy part (unreinforced part) Composite part (V f =45vol%)
10mm
Fig. 4
Macrostructure of vertical cross-section of a specimen.
200 µ m Fig. 6
Vickers Hardness
160 120 80 40 0 10 20 30 40 50 Fiber Volume Fraction, V f (vol%) Fig. 5 Vickers hardness of AC4A alloy and composites plotted versus fiber volume fraction.
rates were 10 K/min, and the applied load was 4:9 102 N. The average coefficient of the thermal expansion (abbreviated CTE) was obtained from the thermal expansion curve gradient. The thermal conductivity and thermal expansion tests were carried out using 5 test pieces per a composite. 3.
Results and Discussion
3.1 Macrostructure and hardness of composites Figure 4 shows the macrostructure of the vertical crosssection of the composite (Vf ¼ 45 vol%). It can be seen that the melt infiltration was perfectly accomplished with no observable defects. The composite had about a 20 mm height, which was almost equal to that of the preform before the infiltration. Based on these results, it can be stated that the infiltration was successful without preform contraction or deformation. The melt infiltration was perfectly accomplished also when Vf was 25 and 35 vol%. Figure 5 shows the Vickers hardness of the AC4A alloy and composites plotted versus Vf . It shows that the hardness increased as Vf increased. The Vickers hardness of the composite with 45 vol% fiber was about 140, and the test pieces were easily machined from the composite using the commercial super alloy or diamond tool. 3.2 Microstructure of composites Figure 6 shows an optical micrograph of the parallel section of the composite with 45 vol% fiber. The dark phases observed in the micrograph are the short potassium titanate fibers. No agglomeration of the fibers or porosity is observed in the composite, indicating that the melt infiltration into the fiber preform was perfectly accomplished.
Optical micrograph of a composite (Vf ¼ 45 vol%, parallel section).
Figure 7 shows the optical micrographs of the composites. The distribution of the aspect ratio of the fibers was measured using 10 fields in the optical micrographs at a magnification of 100 for each composite. When comparing the aspect ratio of the fiber on the vertical section with that on the parallel section, it can be seen that the fibers with the aspect ratio of 1–3 comprise about 50% of the vertical section, while less than 40% of the parallel section. The fibers with a high aspect ratio increased in the parallel section. This tendency was pronounced as Vf increased. The high aspect ratio in the parallel section indicates that many of the fibers are in a random parallel arrangement. As described earlier, there was little difference between the thickness of the preform and that of the composite. Therefore, the infiltration process did not change the original fiber orientation in the preform, i.e., the fiber alignment remained mainly parallel in the plane, but strongly affected the fiber distribution depending on Vf in the preforms. The eutectic silicon that originally existed in the alloy was finely dispersed in the matrix, which appears light gray in the microstructure. 3.3 Thermal conductivity of composites The thermal conductivity of the composites is plotted versus Vf in Fig. 8, showing that thermal conductivity decreased as Vf increased. The decrease in the thermal conductivity is due to the reinforcement with the potassium titanate fibers having a lower thermal conductivity than the AC4A alloy. The thermal conductivity in the P direction was higher than that in the V direction, and this tendency is pronounced as Vf increased. It is considered to be due to the fiber alignment; the fibers aligned parallel to the plane increased as Vf increased, and they inhibited the heat transfer in the V direction. Particularly, in the present study, the thermal conductivity of the fiber (1.7 W/(mK)) is oneninetieth of that of the matrix (155 W/(mK): experimental value). When the difference in their thermal conductivity is high, the difference in the fiber alignment would affect the thermal conductivity behavior even though the difference in the alignment was very small. A three-dimensional random alignment of the fiber would reduce the anisotropy in the thermal conductivity. In Fig. 8, the theoretical values of the thermal conductivity calculated using Landauer’s model13) are shown along with the experimental values. It is reported
Microstructure and Thermal Properties of Squeeze Cast Aluminum Alloy Composite Reinforced with Short Potassium Titanate Fiber
60 40 20 0
Frequency (%)
Fiber
Vf =35vol%
60 40 20 0 1 3 5 7 9 11 13 15 Aspect ratio
1 3 5 7 9 11 13 15 Aspect ratio
60
Frequency (%)
Eutectic Si
(b) Parallel section Frequency (%)
Frequency (%)
Vf =25vol%
(a) Vertical section
40 20 0
60 40 20 0
60
Frequency (%)
Vf =45vol%
Frequency (%)
1 3 5 7 9 11 13 15 Aspect ratio
40 20 0 1 3 5 7 9 11 13 15 Aspect ratio
Thermal Conductivity, λ /Wm−1 K −1
Fig. 7
Experimental (P) Experimental (V) Calculated(Landauer)
100
60
20 0
100 µ m
1 3
5 7 9 11 13 15 Aspect ratio
1 3
5 7 9 11 13 15 Aspect ratio
60 40 20 0
Optical micrographs of the composites and aspect ratio of fibers measured on the micrographs.
180
140
2667
10 20 30 40 50 Fiber Volume Fraction,V f (vol%)
Fig. 8 Experimental and calculated thermal conductivity of the composites plotted versus fiber volume fraction.
that the values calculated using Landauer’s model expressed as eq. (3) for random mixtures of two components agree with the experimental values:13) pffiffiffiffi f3ð1 Vf Þ 1gm þ ð3Vf 1Þf þ D ð3Þ c ¼ 4 where c , f , and m are the thermal conductivities of the composite, fiber, and matrix, respectively. D is calculated using eq. (4):
D ¼ ½f3ð1 Vf Þ 1gm þ ð3Vf 1Þf 2 þ 8m f
ð4Þ
Figure 8 shows that the value calculated using Landauer’s model is nearly the mean between the experimental value in the P and V directions. This indicates that the thermal conductivity of the composite in the present study can be roughly estimated if the fibers are in a three-dimensional random alignment. Furthermore, these results show that the reinforcement with the short potassium titanate fiber decreases the thermal conductivity of the aluminum alloy. 3.4 Thermal expansion behavior of composites Figure 9 shows the effect of Vf and the measuring direction on the thermal strain of the AC4A alloy and composites. Nearly linear dilation characteristics are observed in both the alloy and the composites. The extension of the composite with 45 vol% fiber in the V direction was found after the first cycle. It is considered to be due to the residual stress in the composite generated during the casting process. Generally, when the melt solidifies after the infiltration, the tensile stress in the direction parallel to the fiber and the compressive stress in the direction perpendicular to the fiber are generated due to the difference in the CTE between the fiber and the matrix. The heating during the thermal expansion test removes these stresses, which leads to shrinkage in the direction parallel to the fiber and extension in the direction perpendicular to the fiber. The extension and shrinkage is high when the anisotropy in the fiber distribution
2668
K. Asano, H. Yoneda and Y. Agari
in Fig. 10. In this figure, the theoretical values calculated using the rule of mixture (R.O.M.) expressed as eq. (5) are shown along with the experimental values:
Strain, δ (%)
1.2 0.8
AC4A (Vf =0 vol%)
c ¼ f Vf þ m ð1 Vf Þ
0.4 0
Strain, δ (%)
1.2 0.8
Vf =25vol% V direction
Vf =25vol% P direction
Vf =35vol% V direction
Vf =35vol%
Vf =45vol% V direction
V f =45vol% P direction
0.4 0
Strain, δ (%)
1.2 0.8
P direction
0.4 0
Strain, δ (%)
1.2 0.8 0.4 0 300
500 700 Temperature, T /K
300
500 700 Temperature, T /K
Fig. 9 Effect of fiber volume fraction and measuring direction on thermal strain of AC4A alloy and composites.
22 Experimental(P) Experimental(V) Calculated(R.O.M.)
−6
CTE, α /10 K
−1
20
18
16
14 0
10 20 30 40 50 Fiber Volume Fraction, V f (vol%)
Fig. 10 Experimental and calculated CTE of the composites (320–473 K) plotted versus fiber volume fraction.
is high, and greater in the direction perpendicular to the fiber than parallel to the fiber.14) The extension of the composite with 45 vol% fiber in the V direction after the first cycle would be due to this phenomenon. However, nearly constant dilation characteristics were observed after the second cycle in every composite in both directions. Therefore, the CTE between 320 and 473 K during the second heating process was calculated and plotted versus Vf
ð5Þ
where c , f , and m are the CTE of the composite, fiber, and matrix, respectively. The experimental values decreased as Vf increased. The experimental value in the P direction was slightly lower than that in the V direction for every Vf . Generally, when the matrix is reinforced with the fibers having a low CTE, the CTE is lower in the direction parallel to the fiber than perpendicular to the fiber.15) Since the preforms in which the fibers were aligned parallel to the plane were used in the present study, the CTE in the P direction was lower than that in the V direction. However, the difference in the CTE was quite less than that in the thermal conductivity. The CTE of the fiber (6:8 106 /K) is one-third that of the matrix, and their difference is quite low when compared with the thermal conductivity (one-ninetieth). This would lead to the small effect of the fiber alignment on the CTE. It is concluded that the anisotropy in the thermal expansion is extremely low in the composite used in the present study. Furthermore, the experimental values were in good agreement with the theoretical values calculated using R.O.M., indicating that the CTE of the composite used in the present study can be easily predicted. 4.
Conclusions
The present study has led to the following conclusions. (1) The infiltration of the AC4A aluminum alloy melt into the short potassium titanate fiber preform was perfectly accomplished by the squeeze casting, and the fibers were distributed homogeneously in the alloy. Although the hardness increased as Vf increased, the composite with 45 vol% fiber was easily machined using the commercial cutting tools. (2) The thermal conductivity decreased as Vf increased. Although the thermal conductivity in the P direction was a little higher than that in the V direction, the thermal conductivity of the composite can be roughly estimated using the Landauer’s model. (3) Nearly linear dilation characteristics were observed in both the alloy and the composites. The CTE decreased as Vf increased. The difference between the CTE in the P direction and V direction was slight, and the experimental values were in good agreement with the theoretical values calculated using the R.O.M. Acknowledgement We wish to thank Kubota Co. for the provision of short potassium titanate fibers. REFERENCES 1) Y. D. Huang, N. Hort and K. U. Kainer: Composites Part A 35 (2004) 249–263. 2) W. D. Fei, M. Hu and C. K. Yao: Mater. Chem. Phys. 77 (2003) 882–888.
Microstructure and Thermal Properties of Squeeze Cast Aluminum Alloy Composite Reinforced with Short Potassium Titanate Fiber 3) L. D. Wang, W. D. Fei and C. K. Yao: Mater. Sci. Eng. A A336 (2002) 110–116. 4) K. Suganuma: J. Jpn. Inst. Metals 58 (1994) 1213–1219. 5) H. Harada, Y. Kudoh, Y. Inoue and I. Tsuchitori: J. Jpn. Inst. Metals 58 (1994) 69–77. 6) I. Tsuchitori and H. Fukunaga: J. Jpn. Inst. Metals 56 (1992) 333–341. 7) K. Suganuma, T. Fujita and K. Niihara: J. Jpn. Inst. Metals 54 (1990) 1422–1431. 8) Y. Nishida, T. Imai, M. Yamada, H. Matsubara and I. Shirayanagi: J. JILM 38 (1988) 515–521. 9) H. Yamato, A. Ohgami, I. Akiyama, T. Oyabu, Y. Morimoto, S.
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